Extended Forecast Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Wed Apr 05 2023 Valid 12Z Sat Apr 08 2023 - 12Z Wed Apr 12 2023 ...Overview and Guidance/Predictability Assessment... The latest model runs continue to have low predictability with a series of northern and southern stream shortwave troughs and impulses later this week before re-amplifying with increasing uncertainty next week. The Southeast/Mid-Atlantic region is one of the main locations were the forecast is uncertain beyond the weekend. Some of the models keep higher QPF inland which may increase the risk for local flooding concerns while the rest places the highest amounts offshore which would lower the threat for flooding. The extent of northern and stream interaction/upper trough amplification over the East and low genesis has varied, albeit with a recent guidance trend away from stream phasing aloft. The preferred WPC blend leaned heavily on the GEFS and EC ensemble means beyond Day 4 which followed suite from the previous forecast to maintain continuity. ...Weather/Hazards Highlights... Showers and thunderstorms will track across the Gulf states, Southeast and Mid-Atlantic region through the weekend and then linger near the Eastern Seaboard on Monday as the leading front will track offshore and settle over the Atlantic Ocean and back over the Gulf of Mexico through this period. This setup is shaping up to be favorable for periods of moderate to possibly heavy rainfall that may result in localized flash flooding, at least Saturday into Sunday. There is an experimental Day 4 Marginal Risk for excessive rainfall that spans from southern Alabama to southern North Carolina. The proximity of the wavy front should support some additional precipitation across the Southeast Sunday, lingering mainly over Florida into early next week with renewed low developments. The models are suggesting the higher amounts will be near or just offshore the Mid-Atlantic/Southeast Coast. There is potential for some of these locations to have excessive rainfall however, the guidance has a fair amount of spread in placement and amounts for this period and beyond. With below average predictability, confidence is not high on where or if a Marginal Risk area may be needed for Day 5. Northern stream system progression early-mid next week offers modest precipitation chances, but digs trailing cold fronts down into the central and eastern states. Meanwhile, activity across the Southeast has a growing signal for a separated southern stream upper trough/low with potential for moderate coastal/maritime low developments out over the western Atlantic, but ample uncertainties remain. Multiple shortwaves will move onshore the West Coast during this period as the flow aloft becomes more zonal, favoring moderate precipitation across the Pacific Northwest/northern California this weekend along with modest terrain snows further inland across the Northwest. By the beginning of the new week the zonal flow will transition again as the the upper trough over the Eastern Pacific becomes more amplified while a downstream/warming upper ridge moves in across the West Coast. As the week progresses the trough will shift eastward across the West Coast which will promote an active wet pattern for the Pacific Northwest. Temperatures will trend up across the western states, the Rockies and the adjacent high Plains. A few locations may tie or set new daily records. Campbell Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of California and the Pacific Northwest, Sun, Apr 9. - Heavy rain across portions of the Southeast, the southern Mid-Atlanticand,and the Southern Appalachians, Sat, Apr 8. - Heavy snow across portions of the Pacific Northwest, Sun-Wed, Apr 9-Apr 12. - Heavy snow across portions of the Pacific Northwest, Tue-Wed, Apr 11-Apr 12. - Flooding possible across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Southeast and the Northern Plains. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml